1. Field of Invention
This invention relates generally to a dual charging system of an image forming device.
2. Description of Related Art
One method of printing in multiple colors with a color copier, a color digital copier or a color laser printer is to uniformly charge a charge-retentive surface, such as a photoreceptor belt, and subsequently expose portions of the surface to define information to be reproduced in a first color. This information is rendered visible using chargeable toner particles. The charge-retentive surface is then recharged to a uniform potential and subsequently exposed and developed either at the same image forming station or the next image forming station, if more than one station is used, to form additional color layers.
This recharge, expose and develop (REaD) process is repeated to subsequently develop images of different colors to be superimposed on the surface of the charge-retentive surface before the full color image is transferred to a support substrate, such as paper. The different colors are developed on the charge-retentive surface in an image-on-image (IOI) process. Each different image may be formed by using a single exposure device, where each subsequent color image is formed in a subsequent pass of the charge-retentive surface. Alternatively, each different color image may be formed by multiple exposure devices corresponding to each different color image during a single pass of the charge-retentive surface.
Several issues arise that are unique to the REaD image-on-image process for creating multi-color images when attempting to provide optimum conditions for the development of subsequent color images onto previously-developed color images. For example, during a recharge step, it is important to level the voltages among previously toned and un-toned areas of the charge-retentive surface so that subsequent exposure and development steps are performed across a uniformly charged surface. The greater the difference in voltage between those image areas of the charge-retentive surface previously subjected to a development and recharge step, and those bare non-developed or un-toned areas of the charge-retentive surface, the larger the difference in the development potential can be between these areas for the subsequent development of image layers on the previous layers.
Another issue that must be addressed with the REaD image-on-image color image formation process is the residual charge and the resultant voltage drop that exists across the toner layer of a previously-developed area of the charge-retentive surface. Although it may be possible to achieve a uniform voltage by recharging the previously-toned layer to the same voltage level as the neighboring bare areas, the associated residual toner voltage prevents the effective voltage above any previously-developed toned areas from being re-exposed and discharged to the same level as neighboring bare photoreceptor areas which have been exposed and discharged to the actual desired voltage levels. Furthermore, the residual voltage associated with previously-developed toner images reduces the dielectric and effective development field in the toned areas, which tends to hinder attempts to achieve a desired uniform consistency of the developed mass of subsequent toner images.
These problems become increasingly severe as additional color images are subsequently exposed and developed on the charge retentive surface. Color quality of the final reproduced image is severely threatened by the presence of the toner charge and the resultant voltage drop across the toner layer. The change in voltage due to the toned image can be responsible for color shifts, increased moire, increased color shift sensitivity to image misregistration, and toner spreading at the image edges, thus affecting many of the imaging subsystems. Therefore, it is desirable to reduce, or ideally, eliminate, the residual toner voltage of any previously developed toned images and ensure that the potential difference across each toner layer is consistent and ideally minimum.
One way to improve the consistency of charge levels between the bare charge-retentive surface and previously toned areas is to use a dual recharge system, otherwise known as a split recharge system. In a dual recharge system, an AC charging device is coupled with a DC charging device to apply a charge to the charge-retentive surface. The DC and AC charging devices are set to given charge levels that cause the charge-retentive surface to be charged to a corresponding level. Precision adjustments can be made using the AC charging device. However, the ability of charging devices to consistently charge the charge-retentive surface is difficult to determine because it is at least partially dependent upon machine-specific characteristics, including characteristics of the charging devices themselves, and because these parameters vary with time and use of the image forming machine.
Because both charging devices are running during the image forming process, it is also impossible to isolate and measure the charge on the charge-retentive surface resulting from the DC charging device, since the charge resulting from the DC charging device is masked by the charge resulting from the AC charging device. Moreover, the ability of the DC charging device to charge the charge-retentive surface is based on physical parameters within the charging device, such as spacing to the charge-retentive surface and contamination levels, and is characterized by a linear relationship between the applied grid voltage and the charge-retentive surface charge level measured by the voltage sensing device.